Very Short Delivery times of Meteorites after the L-chondrite Parent Body

Cosmic-ray exposure ages of 480 million year old fossil meteorites found in Southern Sweden constrain delivery times of meteorites from a large collision in the asteroid belt. Chromite grains from seven meteorites from four stratigraphic layers define an exposure age gradient as expected if the meteorites originated from the well-known L-chondrite parent body break-up event. The very short delivery times of a few 10 years for the earliest fragments reaching Earth are in the range predicted by dynamical models. Introduction: Fossil meteorites found in Ordovician sediments in the active Thorsberg quarry in Southern Sweden by Schmitz et al. [1] provide an excellent opportunity to study one of the largest collisions in the asteroid belt in recent solar system history. Abundances of fossil meteorites and sedimentdispersed extraterrestrial chromite grains constrain the lower Ordovician meteorite flux to have been about two orders of magnitude higher than today [2]. Chemical composition of relict chromite grains indicate that the fossil meteorites are L or LL chondrites. Evidence for a break-up event of an L chondrite parent body is mainly given by the fact that many L chondrites have Ar-Ar gas retention ages around 500 Ma [3, 4]. The goal of this study is to determine delivery times of fossil meteorites from different stratigraphic locations in the Thorsberg quarry (~480 Ma). If all fossil meteorites originate from the same parent body disruption event, they should have similar exposure ages. In particular, the ones deposited in older sediments are expected to have had slightly shorter space residence times than the ones found in younger strata. The age difference between the oldest and youngest sediments investigated is about 1 to 2 Myr. Samples and Experimental: We have analyzed helium and neon isotopes in 20 batches of relict chromite grains from seven fossil meteorites found in four different stratigraphic layers in the Thorsberg quarry. We have shown earlier that relict chromite grains in fossil meteorites retain cosmogenic noble gases [5] since this mineral phase has experienced minimal alteration [1]. After grain extraction from the fossil meteorites [1, 5], batches of a few grains (4-40 μg total sample weight) were melted with an IR-laser in ultra-high vacuum to release noble gases. He and Ne isotopes were measured [5] using an ultra-high-sensitivity noble gas mass spectrometer [6]. Exposure ages are determined using average chromite composition as reported by Schmitz et al. [1] and elemental production rates as published by Leya et al. [7] assuming “average” shielding. Uncertainties of exposure ages are ~30-40% not taking into account uncertainties due to unknown shielding of samples [5]. Results: Cosmogenic He and Ne are detected in all 20 samples. Terrestrially-produced cosmogenic nuclide contributions (<10 %) can be safely neglected. Nominal Ne exposure ages (T21) of all meteorites are corrected for nucleogenic neon (Nenuc). Nucleogenic Ne (assuming Nenuc/Herad = 2.8 · 10 [8]) for all but one meteorite account for less than ~3% of the Ne excess over atmospheric composition. In one sample of the meteorite Arkeologen (Ark) 007 Nenuc nominally comprises about 28% of the excess Ne. This samples contains not only relatively large amounts of nucleogenic Ne but also has a Heconcentration at least one order of magnitude higher than the other samples, all of which is assumed to be radiogenic. This implies large U and Th uptake during alteration on Earth. Since we determine Nenuc from He concentrations, and helium is more readily lost than neon, the corrections for Necos may represent a lower limit. The correction for the Ark 007 samples is very high and their T21 is thus probably not real. T21 of different meteorites from the same quarry beds agree within 2σ (Figure 1). T21 are a few 10 years to ~ 1 Myr and are unusually low compared to ordinary chondrites falling today (cf. [9]). Very remarkably, a T21 gradient is observed through the sediment column, meteorites from younger sediments showing higher ages. The T21 range of ~1 Myr is in accordance with the biostratigraphically and sedimentologically constrained age range of 1-2 Myr. Nominal He exposure ages (T3) do not show this range nor a gradient. This implies helium from some samples has been lost, thus T3 are not considered to reflect true space residence times and are therefore not shown. Discussion: Low nominal exposure ages might be a result of overestimated production rates. This would require that all samples were considerably more heavily shielded than assumed, ie. are fragments of very large meteoroids. This is highly unlikely. The span of exposure ages is within a factor of two of the sediment deposition period. We therefore conclude all Necos has been retained quantitatively or nearly so, possible Lunar and Planetary Science XXXV (2004) 1492.pdf